1. Field of the Invention
The present invention relates to a DC-DC converter and an electronic device including such a DC-DC converter. More particularly, the present invention relates to a DC-DC converter in which switching loss is reduced and to an electronic device including such a DC-DC converter.
2. Description of the Related Art
FIG. 6 is a circuit diagram of a step-down DC-DC converter. In FIG. 6, the DC-DC converter 1 includes a DC power supply Vin, a rectifier diode D1, a choke coil L1, a MOSFET Q1 functioning as a switching element, a smoothing capacitor C1, a diode D2, a capacitor C2, a capacitor C3, and a control circuit 2. The diode D2 is a body diode of the MOSFET Q1, and the capacitor C2 is a drain-source junction capacitance, that is, a parallel capacitance, of the MOSFET Q1. The capacitor C3 is an anode-cathode junction capacitance, that is, a parallel capacitance, of the rectifier diode D1.
The cathode of the rectifier diode D1 is connected to one end of the choke coil L1 and the anode thereof is grounded. The source of the MOSFET Q1 is connected to the node between the rectifier diode D1 and the choke coil L1 and the drain thereof is connected to one end of the DC power supply Vin. The other end of the DC power supply Vin is grounded. The other end of the choke coil L1 is connected to an output terminal Po. The smoothing capacitor C1 is connected between the output terminal Po and the ground. The control circuit 2 is connected between the output terminal Po and the gate, which is the control terminal, of the MOSFET Q1.
Now, the operation of the DC-DC converter 1 will be described. The control circuit 2 ON/OFF-drives the MOSFET Q1. First, when the MOSFET Q1 is ON, a current flows to the choke coil L1 through the MOSFET Q1 by an input voltage supplied from the DC power supply Vin. When the MOSFET Q1 is turned OFF, a current flows to the choke coil L1 through the rectifier diode D1 due to the excitation inertia of the choke coil L1. By repeating this operation, a voltage according to the duty of the ON/OFF operation of the MOSFET Q1 is output from the output terminal Po. The control circuit 2 changes the duty of switching of the MOSFET Q1 according to an output voltage in order to perform PWM control so that the output voltage is kept constant regardless of variations in the input voltage and a load.
FIG. 7 is a circuit diagram of a step-up DC-DC converter. In FIG. 7, elements which are the same as those in FIG. 6 are denoted by the same reference numerals. In FIG. 7, the DC-DC converter 5 includes a DC power supply Vin, a rectifier diode D3, a choke coil L2, a MOSFET Q2 functioning as a switching element, a smoothing capacitor C1 a diode D4, a capacitor C4, a capacitor C5, and a control circuit 2. The diode D4 is a body diode of the MOSFET Q2, and the capacitor C4 is a drain-source junction capacitance, that is, a parallel capacitance, of the MOSFET Q2. The capacitor C5 is an anode-cathode junction capacitance, that is, a parallel capacitance, of the rectifier diode D3.
The anode of the rectifier diode D3 is connected to one end of the choke coil L2 and the cathode thereof is connected to the output terminal Po. The drain of the MOSFET Q2 is connected to the node between the rectifier diode D3 and the choke coil L2 and the source thereof is grounded. The other end of the choke coil L2 is connected to one end of the DC power supply Vin. The other end of the DC power supply Vin is grounded. The smoothing capacitor C1 is connected between the output terminal Po and the ground. The control circuit 2 is connected between the output terminal Po and the gate, which is the control terminal, of the MOSFET Q2.
Now, the operation of the DC-DC converter 5 will be described. The control circuit 2 ON/OFF-drives the MOSFET Q2. First, when the MOSFET Q2 is ON, a current flows to the choke coil L2 and then to the MOSFET Q2 by the input voltage from the DC power supply Vin so that the choke coil L2 is excited. When the MOSFET Q2 is OFF, a current flows from the DC power supply Vin through the choke coil L2 and the rectifier diode D3. At this time, the voltage at one end of the choke coil L2 is higher than that at the other end thereof because of its inertia. Therefore, when the voltage at the other end of the choke coil L2 reaches the input voltage Vin, the voltage at the one end surpasses the input voltage Vin, and thus a step-up operation is realized. Then, a voltage that is increased by repeating this operation is output from the output terminal Po. As in the DC-DC converter 1, the control circuit 2 changes the duty of switching of the MOSFET Q2 according to an output voltage in order to perform PWM control so that the output voltage is kept constant regardless of variations in the input voltage and a load.
When the switching element of the DC-DC converter is ON, a current is applied to the switching element but an ON-resistance is almost zero and thus, almost no loss is caused. On the other hand, when the switching element is OFF, a voltage is applied to the switching element but a current is not applied thereto, and thus, almost no loss is caused.
However, in the DC-DC converters 1 and 5, when the MOSFET Q1 or Q2 functioning as a switching element is turned ON/OFF, a voltage and a current are applied to the switching element for a moment, and large switching loss is caused at that time. Further, the current flowing through the MOSFET Q1 or Q2 and the rectifier diode D1 or D3 abruptly changes and thus, a high noise may be generated. Also, when the MOSFET Q1 or Q2 is turned ON, a surge recovery current flows from the cathode to the anode during a reversed recovery time of the rectifier diode D1 or D3, which leads to great loss.
In order to overcome this problem, Japanese Patent No. 3055121 discloses a configuration for realizing zero-voltage switching and zero-current switching of a switching element by using resonance.
In this configuration, switching loss and noise can be reduced. However, a capacitance that is large enough to supply a load current is required as a resonance capacitor. Accordingly, a resonance period depending on a resonance capacitor and a resonance reactor is necessary at the time when the switching element (switching element 2) is turned ON/OFF. Thus, PWM control, in which ON-period and OFF-period of the switching element is further shortened, is not performed. As a result, a wide-range variation in the input voltage and output voltage are not adequately dealt with. Further, a sine-wave resonance current is added to the output current flowing through the switching element. Therefore, a switching element having a large current capacitance is required, which leads to an increase in the size and cost of the DC-DC converter.
In order to overcome the problems described above, preferred embodiments of the present invention provide a DC-DC converter in which switching loss and noise are greatly reduced, a wide-range variation in an input voltage and output voltage is dealt with, and an increase in the size and cost is prevented, and also provide an electronic device including such a novel DC-DC converter.
According to a preferred embodiment of the present invention, a DC-DC converter includes a rectifier diode, a choke coil, one end thereof being connected to one end of the rectifier diode, a first switching element, one end thereof being connected to the node between the rectifier diode and the choke coil through a resonance coil, a first diode connected in parallel to the first switching element, a second switching element, a series circuit including a capacitor and the second switching element and connected in parallel to a series circuit including the resonance coil and the rectifier diode, and a second diode connected in parallel to the second switching element. Each of the first and second switching elements and the rectifier diode includes a parallel capacitance between terminals thereof.
The first and second switching elements are alternately turned ON, and a period when both switching elements are OFF is provided between the ON periods.
In a period when the rectifier diode conducts, the sum of a current flowing through the rectifier diode and a current flowing through the resonance coil flows through the choke coil.
The other end of the first switching element is connected to one end of a DC power supply, the other end of the choke coil is connected to an output terminal, and the other end of the rectifier diode is connected to the other end of the DC power supply, whereby a step-down operation is performed. Alternatively, the other end of the choke coil is connected to one end of a DC power supply, the other end of the rectifier diode is connected to an output terminal, and the other end of the first switching element is connected to the other end of the DC power supply, whereby a step-up operation is performed.
The DC-DC converter may further include a third diode connected in parallel to a series circuit including the first switching element and the resonance coil.
An electronic device of another preferred embodiment of the present invention includes the above-described DC-DC converter.
With this configuration, in the DC-DC converter of various preferred embodiments of the present invention, loss and noise are greatly reduced.
Also, in the electronic device of another preferred embodiment of the present invention, power consumption and noise are greatly reduced.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.